Race cars often don't have a speedometer. What they do have is a tachometer that shows how fast the engine is revving.

A heart rate monitor is a tachometer for the human body -- it tells a user how quickly the heart is beating during exercise. Sports- and fitness-related use of the device has mushroomed since its invention in 1977, but many people don't know how to use one effectively. For starters, simple formulas to determine heart rate training zones aren't reliable. A monitor should be used not just to achieve a target, but as a regulator to avoid overtraining. The devices, although generally accurate, can sometimes generate false readings that cause unnecessary worry. And for the easily obsessed, focusing too much on numbers can turn a fun bike ride into a laboratory experiment.

Most monitors have a chest strap sensor that picks up and transmits electrical impulses from the heart to a receiver, worn on the wrist or handlebars of a bike. The receiver converts signals from the strap to a digital display of beats per minute. Strapless models are also available. Basic monitors cost less than $100 and display heart rate and time. Fancier models record heart rates during a workout and can set alarms for high and low heart rate zone limits. Other models integrate bike computer functions, such as speed and distance, or use GPS satellite signals to determine speed, distance, location and altitude in addition to recording heart rate.

 

"A heart rate monitor allows an individual to make any activity into a healthful and beneficial exercise, such as walking the dog or even cleaning your house, because you can modulate your effort to achieve the exercise effect," says Jose Maresma-Fois, an exercise physiologist with heart rate monitor manufacturer Polar Electro.

The health benefits of exercise aren't gained without enough intensity. The American College of Sports Medicine and the American Heart Assn. suggest that healthy individuals under 65 do moderately intense cardio exercise 30 minutes a day, five days a week or vigorously intense cardio 20 minutes a day, three days a week, in addition to strength training and stretching.

Determining what is "moderately intense" or "vigorously intense" is the rub. That's where heart rate monitors and training zones come in.

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Finding a training zone

Most gyms have one on the wall -- a multihued chart that shows a single heart rate training zone based on age. The zones are derived from a commonly used formula to estimate a person's maximum heart rate: 220 minus age. That rate is genetically fixed for a given activity and declines about one beat per year with age (eventually reaching zero).

But experts say these charts are oversimplified and that for many people they are simply wrong.

"There are plenty of exceptions to that rule, both above and below," says cardiologist and cyclist Dr. John Ellis of Thousand Oaks. "It's a very general guideline, and your results may vary."

A person's maximum heart rate can differ by as many as 30 beats per minute from what the formula produces. Maximum heart rate also varies by activity. So to make heart rate monitors useful, exercisers first need to determine their maximum heart rate for each activity. Runners can establish their actual maximum heart rate during stress tests in which a doctor monitors heart rhythms at increasing exercise levels on a treadmill until the heart rate stops increasing. There is also a do-it-yourself method.

Joe Friel, a coach and author of 10 books about training for endurance athletes, says the truly motivated can determine their maximum heart rate by doing a series of intense intervals while cycling or running. This is like a stress test, except there are no doctors, nurses or crash cart handy if there are problems. After warming up, performing a series of three all-out uphill efforts of two minutes each with 30-second rests will usually peg the heart rate.

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Monitoring intensity

For the weekend athlete, a heart rate monitor can be as much a governor as a goad to keep from overexercising.

"Weekenders often overtrain," says Maresma-Fois. "Too many people think they have to train till they puke. It's in vogue right now to do high-intensity work to burn calories. That's fine, but too many people go out and injure themselves."

Cyclist Steve Gottschalk, 54, rides in the San Fernando Valley and Ventura County wearing a heart rate monitor. "I'm not a young chicken anymore," he says. "I pay attention to my heart rate. If I get above a certain level, I back off."

Endurance athletes and coaches use heart rate monitors that record training sessions to fine-tune training programs.